JP6842260B2 - Image forming device and control program - Google Patents

Description

The present invention relates to an image forming apparatus and a control program, and more particularly to an image forming apparatus and a control program having a double-sided printing function and thermally fixing a toner image formed on an image carrier after being transferred to paper.

Patent Document 1 discloses an example of a conventional image forming apparatus having a double-sided printing function. In an electrophotographic image forming apparatus, an image is formed on paper through a process of charging, exposure, development, transfer, and heat fixing. Here, when the toner image (transferred image) transferred on the paper is heat-fixed, a phenomenon occurs in which the paper shrinks due to the heat. For this reason, when double-sided printing is performed, there is a problem that the size of the image is different between the front side and the back side due to the shrinkage of the paper at the time of heat fixing on the front side (position shift). ..

Therefore, in the technique of Patent Document 1, the image sizes of the front and back surfaces are matched by reducing the transfer image on the back surface side based on the shrinkage rate of the paper by the heat fixing treatment. Specifically, the transfer image on the back surface side is reduced in the sub-scanning direction by increasing the rotation speed of the polygon mirror during back surface printing as compared with that during front surface printing. On the other hand, if the rotation speed of the polygon mirror is increased, the transferred image is enlarged in the main scanning direction. ), The transfer image on the back surface side is reduced in the main scanning direction.

Japanese Unexamined Patent Publication No. 2003-262991

However, in the technique of Patent Document 1, since the rotation speed of the polygon mirror is changed between the front side printing and the back side printing in order to match the image sizes of the front and back sides, it takes time for the rotation of the polygon mirror to stabilize. This is necessary, and printing performance (printing productivity) is reduced.

In order to reduce and form the transferred image on the back surface side without changing the rotation speed of the polygon mirror, there is also a method of reducing the image data by image processing. However, since this reduction process (compression process) is executed by thinning out a part of the image data, there is a risk that the formed image may deteriorate in image quality such as omission of fine lines. In order to prevent thin line omission, a high dpi surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) or the like may be used, but VCSEL is very expensive and costs up.

Therefore, a main object of the present invention is to provide a novel image forming apparatus and control program.

Another object of the present invention is to provide an image forming apparatus and a control program capable of preventing thin line omission and appropriately adjusting the image sizes of the front and back surfaces without reducing the printing productivity.

The first invention is an image forming apparatus having a double-sided printing function, transferring a toner image formed on an image carrier onto paper and then thermally fixing the image, having a laser emitting portion and a polygon mirror, and image data. An exposed unit that forms an electrostatic latent image on the image carrier by scanning the light beam based on, an image processing unit that can enlarge the image data at least in the sub-scanning direction, and surface printing when double-sided printing is performed. The transfer image on the front side is larger than the transfer image on the back side according to the shrinkage rate of the paper due to the heat fixing process without changing the rotation speed of the polygon mirror between the time and the time of printing on the back side. In addition, in the sub-scanning direction, the image data is enlarged by the image processing unit, and in the main scanning direction, the printing clock frequency is changed, so that the image size on the front side and the back side of the paper is changed. The control unit is provided with a control unit that matches the image on the back side based on the set value of the enlargement ratio of the image size on the front side and the difference in the shrinkage ratio of the paper between the front side printing and the back side printing by the heat fixing process. This is a case where the enlargement ratio of the image size on the front side is arbitrarily set based on the enlargement ratio of the size and the set value when the magnification of the image processing in the sub-scanning direction at the time of backside printing is set to 1x or enlargement. However, it is an image forming apparatus that sets the rotation speed of the polygon mirror to the same value between the front side printing and the back side printing.

In the first invention, the image forming apparatus is an image forming apparatus having a double-sided printing function, and forms an image on the paper by transferring the toner image formed on the image carrier to the paper and then heat-fixing the toner image. To do. This image forming apparatus includes an exposure unit, an image processing unit, and the like. When double-sided printing is performed, the control unit of the image forming apparatus enlarges the image data by the image processing unit in the sub-scanning direction, and changes the frequency of the printing clock in the main scanning direction. Match the image sizes of the front side and back side of the paper after heat fixing treatment. That is, the image size of the front and back sides of the paper after the heat fixing process without changing the rotation speed of the polygon mirror between the front side printing and the back side printing, and without using the image data reduction process (compression process). I try to match. In addition, the control unit of the image forming apparatus determines the rotation speed of the polygon mirror during double-sided printing, the set value of the enlargement ratio of the image size on the front side, and the printing of the front side and the back side by the heat fixing process. The image size on the front side is based on the enlargement ratio of the image size on the back side based on the difference in shrinkage ratio and the set value when the magnification of image processing in the sub-scanning direction during backside printing is set to 1x or enlargement. Even if the enlargement ratio of is set arbitrarily, the same value is set between front side printing and back side printing.

According to the first invention, since the rotation speed of the polygon mirror is not changed between the front side printing and the back side printing and the reduction process is not used, it is possible to prevent fine line omission and reduce the printing productivity. The image sizes on the front and back can be adjusted appropriately. In addition, since the enlargement ratio of the image due to image processing can be suppressed to a low level, deterioration of image quality can be suppressed.

The second invention is subordinate to the first invention, and the control unit sets the enlargement ratio of the image size on the front surface side based on the selection operation by the user.

According to the second invention, the user can arbitrarily select the enlargement ratio of the image size on the front surface side.

The third invention is subordinate to the first invention and includes a paper discriminating unit for discriminating the type of paper, and the control unit has an image size on the front surface side based on the type of paper discriminated by the paper discriminating unit. Set the magnification.

According to the third invention, the enlargement ratio of the image size on the front surface side is appropriately changed based on the type of paper.

A fifth invention comprises an exposed portion that forms an electrostatic latent image on an image carrier, and a toner image formed on the image carrier based on the electrostatic latent image is transferred to paper and then heat-fixed on both sides. A control program executed by a computer of an image forming apparatus having a printing function, which is an image processing unit capable of enlarging image data at least in the sub-scanning direction, and when double-sided printing, front-side printing and back-side printing. Sub-scanning so that the transferred image on the front side is larger than the transferred image on the back side according to the shrinkage rate of the paper due to the heat fixing process without changing the rotation speed of the polygon mirror with time. A control unit that matches the image size between the front side and the back side of the paper by enlarging the image data in the direction and by changing the frequency of the print clock in the main scanning direction. The control unit expands the image size on the back side based on the set value of the enlargement ratio of the image size on the front side and the difference in the shrinkage ratio of the paper between the front side printing and the back side printing by the heat fixing process. Even if the enlargement ratio of the image size on the front side is arbitrarily set based on the ratio and the set value when the magnification of image processing in the sub-scanning direction during backside printing is set to 1x or enlargement, the front surface This is a control program that sets the rotation speed of the polygon mirror to the same value during printing and backside printing.

According to the present invention, since the rotation speed of the polygon mirror is not changed between the front side printing and the back side printing and the reduction process is not used, fine line omission can be prevented and the printing productivity is not reduced. The image size of the front and back can be adjusted appropriately.

The above-mentioned object, other object, feature and advantage of the present invention will become more apparent from the detailed description of the examples described below with reference to the drawings.

It is a schematic diagram which shows the schematic structure of the image forming apparatus which is 1st Example of this invention. FIG. 5 is a plan view schematically showing an exposed portion included in the image forming apparatus of FIG. 1. It is a side view which shows typically the exposed part of FIG. 2 together with a photoconductor drum. It is a block diagram which shows the electrical structure of the image forming apparatus of FIG. It is a flow chart which shows an example of the double-sided printing process executed by the CPU of FIG.

[First Example]
With reference to FIG. 1, the image forming apparatus 10 according to an embodiment of the present invention is an electrophotographic image forming apparatus having a double-sided printing function, and performs processes of charging, exposure, development, transfer, and heat fixing. Through the process, a multicolor or monochromatic image is formed (printed) on the paper (recording medium).

In this first embodiment, the image forming apparatus 10 is a color multifunction device (MFP: Multifunction Peripheral) having a copying function, a printer function, a scanner function, a facsimile function, and the like. However, the image forming apparatus 10 does not have to be limited to the multifunction device, and may be any of a copying machine, a printer, and a facsimile. Further, the image forming apparatus 10 may be a monochrome machine.

Further, as will be described later, in the present invention, the direction in which the light beam is scanned by the polygon mirror 82, that is, the axial direction of the photoconductor drum (image carrier) 36 is referred to as the main scanning direction. Therefore, the lateral direction of the paper, which is the direction orthogonal to the paper transporting direction, corresponds to the main scanning direction. On the other hand, the moving direction of the photoconductor drum 36, which is a direction orthogonal to the main scanning direction, is called a sub-scanning direction. Therefore, the vertical direction of the paper, which is the direction in which the paper is conveyed, corresponds to the sub-scanning direction.

First, the basic configuration of the image forming apparatus 10 will be schematically described. As shown in FIG. 1, the image forming apparatus 10 includes an apparatus main body 12 including an image forming unit 30 and the like, and an image reading apparatus 14 arranged above the apparatus main body 12.

The image reading device 14 includes a document mounting table 16 formed of a transparent material. A document holding cover 18 can be opened and closed above the document mounting table 16 via a hinge or the like. The document holding cover 18 is provided with an ADF (automatic document feeder) 24 that automatically feeds the documents placed on the document placing tray 20 one by one to the image reading position 22. Further, on the front side of the document mounting table 16, an operation unit 46 (see FIG. 4) such as a touch panel and operation buttons for receiving input operations such as printing instructions by the user is provided.

Further, the image reading device 14 includes an image reading unit 26 including a light source, a plurality of mirrors, an imaging lens, a line sensor, and the like. The image reading unit 26 exposes the surface of the document with a light source, and guides the reflected light reflected from the surface of the document to the imaging lens by a plurality of mirrors. Then, the reflected light is imaged on the light receiving element of the line sensor by the imaging lens. The line sensor detects the brightness and chromaticity of the reflected light imaged on the light receiving element, and generates image data based on the image of the surface of the document. As the line sensor, a CCD (Charge Coupled Device), a CIS (Contact Image Sensor), or the like is used.

The device main body 12 includes a control unit 28 including a CPU 100 and the like, an image forming unit 30, and the like. The control unit 28 transmits control signals to each part of the image forming apparatus 10 including the exposure apparatus (exposure unit) 32 in response to an input operation to the operation unit 46, and executes various operations to the image forming apparatus 10. Let me.

The image forming unit 30 includes an exposure device 32, a developer 34, a photoconductor drum 36, a photoconductor cleaner unit 38, a charger 40, a transfer unit 42, a fixing unit 44, and the like, and includes a paper feed tray 48 or a manual paper feed tray 50. An image is formed on the paper conveyed from the paper, and the image-formed paper is discharged to the output tray 52. As the image data for forming an image on the paper, the image data read by the image reading unit 26, the image data transmitted from an external computer, or the like is used.

The image data handled by the image forming apparatus 10 corresponds to four color images of black (K), cyan (C), magenta (M), and yellow (Y). For this reason, the developer 34, the photoconductor drum 36, the photoconductor cleaner unit 38, and the charger 40 are each provided with four so as to form four types of electrostatic latent images corresponding to each color. Two image stations are configured.

The photoconductor drum 36 is an image carrier in which a photosensitive layer is formed on the surface of a conductive cylindrical substrate, and the charger 40 is a member that charges the surface of the photoconductor drum 36 to a predetermined potential. is there. Further, the exposure apparatus 32 is configured as a laser scanning unit including a laser diode (LD) 72, a polygon mirror 82, and the like, and is arranged below the photoconductor drum 36, as will be described in detail later. The exposure apparatus 32 exposes the surface of the charged photoconductor drum 36 to form an electrostatic latent image corresponding to the image data on the surface of the photoconductor drum 36. The developing device 34 visualizes the electrostatic latent image formed on the photoconductor drum 36 with four-color (YMCK) toner. Further, the photoconductor cleaner unit 38 removes the toner remaining on the surface of the photoconductor drum 36 after development and image transfer.

The transfer unit 42 includes an intermediate transfer belt 54, a drive roller 56, a driven roller 58, four intermediate transfer rollers 60, a secondary transfer roller 62, and the like, and is arranged above the photoconductor drum 36. The transfer unit 42 does not necessarily have to be provided with the intermediate transfer belt 54, and a configuration in which the toner image on the photoconductor drum 36 is directly transferred to the paper can be adopted.

The intermediate transfer belt 54 is a flexible, endless belt, and is formed of a synthetic resin, rubber, or the like appropriately blended with a conductive material such as carbon black. The intermediate transfer belt 54 is suspended by the driving roller 56 and the driven roller 58, and is arranged so that the outer peripheral surface thereof abuts on the outer peripheral surface of the photoconductor drum 36. Then, the intermediate transfer belt 54 orbits in a predetermined direction as the drive roller 56 is rotationally driven.

The drive roller 56 is rotatably provided around its axis by a drive unit (not shown). The driven roller 58 rotates as the intermediate transfer belt 54 orbits moves, and applies a constant tension to the intermediate transfer belt 54 to prevent the intermediate transfer belt 54 from loosening.

The intermediate transfer roller 60 is arranged at a position facing each photoconductor drum 36 with the intermediate transfer belt 54 interposed therebetween. At the time of image formation, a predetermined voltage (primary transfer voltage) is applied to the intermediate transfer roller 60 to form a transfer electric field between the photoconductor drum 36 and the intermediate transfer belt 54. Then, by the action of this transfer electric field, the toner image formed on the outer peripheral surface of the photoconductor drum 36 is transferred to the outer peripheral surface of the intermediate transfer belt 54.

The secondary transfer roller 62 is provided so as to press the intermediate transfer belt 54 with the drive roller 56. At the time of image formation, a predetermined voltage (secondary transfer voltage) is applied to the secondary transfer roller 62, so that a transfer electric field is formed between the intermediate transfer belt 54 and the secondary transfer roller 62. Then, due to the action of this transfer electric field, the toner image formed on the outer peripheral surface of the intermediate transfer belt 54 is transferred to the paper while the paper passes through the transfer nip region between the intermediate transfer belt 54 and the secondary transfer roller 62. Transcription (secondary transcription).

The fixing unit 44 includes a heat roller 64 and a pressure roller 66, and is arranged above the secondary transfer roller 62. The heat roller 64 is set to have a predetermined fixing temperature, and when the paper passes through the nip area between the heat roller 64 and the pressure roller 66, the toner image transferred to the paper is melted. The toner image is heat-fixed to the paper by mixing and pressure welding.

In such an apparatus main body 12, the paper from the paper feed tray 48 or the manual paper feed tray 50 is sent to the paper output tray 52 via the registration roller 68, the secondary transfer roller 62, and the fixing unit 44. 1 Paper transport path L1 is formed. Further, when performing double-sided printing on paper, the paper after printing on the front side is completed and has passed through the fixing unit 44 is placed on the upstream side of the secondary transfer roller 62 in the paper transport direction as the first paper transport path. A second paper transport path L2 for returning to L1 is formed. A plurality of transport rollers 70 for additionally applying a propulsive force to the paper are appropriately provided in the first paper transport path L1 and the second paper transport path L2.

When single-sided printing is performed on the apparatus main body 12, the paper is guided one by one from the paper feed tray 48 or the manual paper feed tray 50 to the first paper transport path L1 and is transported to the registration roller 68 by the transport roller 70. .. Then, the resist roller 68 conveys the paper to the secondary transfer roller 62 (transfer nip portion) at the timing when the tip of the paper and the tip of the image information on the intermediate transfer belt 54 match, and the toner image is transferred onto the paper. Will be done. After that, by passing through the fixing unit 44 (fixing nip portion), the unfixed toner on the paper is melted and fixed by heat, and the paper is discharged onto the paper ejection tray 52.

On the other hand, when double-sided printing is performed, when the printing on the front side is completed and the rear end of the paper that has passed through the fixing unit 44 reaches the transport roller 70 near the output tray 52, the transport roller 70 is reversed. By rotating the paper, the paper runs in the reverse direction and is guided to the second paper transport path L2. The paper guided to the second paper transport path L2 is conveyed by the transport roller 70 through the second paper transport path L2, and is guided to the first paper transport path L1 on the upstream side of the resist roller 68 in the paper transport direction. At this point, the front and back sides of the paper are reversed, and then the paper passes through the secondary transfer roller 62 and the fixing unit 44, so that printing is performed on the back side of the paper.

Next, the configuration of the exposure apparatus 32 will be described with reference to FIGS. 2 and 3. The exposure device 32 guides light beams emitted from each of the four laser diodes 72 provided for each color to each photoconductor drum 36 via a polygon mirror 82 or the like, and each light beam leads to each photoconductor drum 36. By scanning the image data, an electrostatic latent image corresponding to the image data is formed on the surface of each photoconductor drum 36.

In this first embodiment, the exposure apparatus 32 includes four laser diodes 72, four comeryta lenses 74, four first mirrors 76, a cylindrical lens 78, a second mirror 80, a polygon mirror 82, and a polygon motor (PM). It includes 84, a first fθ lens 86, a second fθ lens 88, a plurality of third mirrors 90, four third fθ lenses 92, and the like. However, the specific configuration of the exposure device 32 is not limited to this, and can be changed as appropriate.

The laser diode 72 is a laser emitting portion that emits a laser beam for exposing the surface of the photoconductor drum 36, and is provided for each color. Each comerita lens 74 converts the light beam emitted from each laser diode 72 into parallel light. Further, the first mirror 76 is arranged so as to guide each of the light beams transmitted through each comerita lens 74 to the cylindrical lens 78.

The cylindrical lens 78 is a lens having an optical power in one direction, and condenses and converges each light beam in the sub-scanning direction, and emits each light beam as it is as parallel light in the main scanning direction. The second mirror 80 reflects the laser beam that has passed through the cylindrical lens 78 and guides it to the polygon mirror 82.

The polygon mirror 82 has a plurality of mirror surfaces (six surfaces in this embodiment) on its outer peripheral surface, and rotates with the rotational drive of the polygon motor 84. Then, by reflecting each laser beam on each mirror surface, each laser beam is repeatedly deflected in the main scanning direction.

The first fθ lens 86 mainly transmits each light beam deflected at an equal angular velocity in the main scanning direction by the iso-angular velocity motion of the polygon mirror 82 at a uniform velocity along the main scanning line on each photoconductor drum 36. Convert to move. Further, the second fθ lens 88 mainly converts each light beam from the polygon mirror 82, which is diffuse light, into parallel light in the sub-scanning direction, and from the polygon mirror 82, which is parallel light in the main scanning direction. Each light beam is focused and emitted so as to have a predetermined beam diameter (spot diameter) on the surface of each photoconductor drum 36.

Further, each of the third mirrors 90 is arranged so as to guide each light beam transmitted through the first fθ lens 86 and the second fθ lens 88 to each of the corresponding third fθ lenses 92. The third fθ lens 92 mainly condenses each light beam which is parallel light in the sub-scanning direction and squeezes it so as to have a predetermined beam diameter on the surface of the corresponding photoconductor drum 36, and also in the main scanning direction. , Each light beam converged by the second fθ lens 88 is emitted to the corresponding photoconductor drum 36 as it is.

In such an exposure apparatus 32, each light beam emitted from the laser diode 72 is reflected and deflected by the mirror surface of the polygon mirror 82, and is incident on each photoconductor drum 36 through each optical path, and each The surface of the photoconductor drum 36 is repeatedly main scanned. On the other hand, since each photoconductor drum 36 is rotationally driven, a two-dimensional electrostatic latent image is formed on the surface of each photoconductor drum 36.

Subsequently, with reference to FIG. 4, the electrical configuration of the image forming apparatus 10 will be briefly described. As shown in FIG. 4, the image forming apparatus 10 includes a control unit 28 composed of a CPU 100, a RAM 102, a ROM 104, and the like. The PM driver 108, the LD data generation unit 110, the image processing unit 112, the operation unit 46, and the like are connected to the CPU 100 via the bus 106. Further, the PM driver 108 is connected to the polygon motor 84, and the LD data generation unit 110 is connected to the laser diode 72.

Although not shown, the CPU 100 is appropriately connected to the CPU 100 via a bus 106 with a process unit including the photoconductor drum 36 and other parts, and other parts of the image forming apparatus 10 such as the transfer unit 42 and the fixing unit 44. To.

The CPU 100 controls the overall control of the image forming apparatus 10, and comprehensively controls the operation of each part of the image forming apparatus 10. The RAM 102 is used as a work area and a buffer area of the CPU 100. The ROM 104 is the main storage device of the image forming apparatus 10, and appropriately stores a control program, data, and the like for the CPU 100 to control the operation of each part of the image forming apparatus 10. As an example, the ROM 104 stores a double-sided printing program for controlling the LD data generation unit 110, the image processing unit 112, and the like to match the image sizes of the front side and the back side of the paper after the heat fixing process. .. The CPU 100 writes (stores) the control program and data read from the ROM 104 into the RAM 102, and executes control of each part constituting the image forming apparatus 10 according to the control program stored in the RAM 102.

The PM driver (polygon motor control unit) 108 controls the rotation speed of the polygon motor 84, that is, the rotation speed of the polygon mirror 82, based on the control signal output from the CPU 100. If the rotation speed of the polygon mirror 82 is increased while other conditions such as the process speed are constant, the speed at which the laser beam mainly scans on the photoconductor drum 36 increases, so that the polygon mirror 82 is formed on the photoconductor drum 36. The size of the electrostatic latent image (which in turn is the transferred image which is the toner image transferred to the paper) is enlarged in the main scanning direction and reduced in the sub-scanning direction. On the contrary, when the rotation speed of the polygon mirror 82 is reduced, the size of the electrostatic latent image formed on the photoconductor drum 36 is reduced in the main scanning direction and enlarged in the sub-scanning direction.

The LD data generation unit 110 generates a drive signal for blinking the laser diode 72 according to the image data and the print clock (also referred to as a print clock or a pixel clock) based on the control signal output from the CPU 100. In this first embodiment, the frequency or speed of the print clock can be finely adjusted by an f-PLL (fractional PLL) or the like. When the frequency of the print clock is increased (increased in speed) while keeping other conditions constant, the width in the main scanning direction per pixel becomes smaller, so that the electrostatic latent image formed on the photoconductor drum 36 is formed. Is reduced in the main scanning direction and enlarged in the secondary scanning direction. On the contrary, when the frequency of the print clock is reduced (decreased), the electrostatic latent image formed on the photoconductor drum 36 is enlarged in the main scanning direction and reduced in the sub-scanning direction.

The image processing unit 112 is a processing means for executing image processing of image data such as enlargement / reduction processing, density conversion processing, and data format (file format) conversion processing. In this first embodiment, at least the image data is subordinated. Enlargement processing is possible in the scanning direction. Further, the image processing unit 112 can also handle image data after halftone adjustment. That is, it is possible to cope with the case of reprinting the image data after the halftone, or the case of receiving the image data after the halftone from an external computer and printing it. As a method for enlarging the image data, known algorithms such as linear interpolation method, nearest neighbor interpolation method and cubic convolution interpolation method can be used. For example, it is preferable that the image data is enlarged by using the image processing method proposed by the inventors of the present application in Japanese Patent Application Laid-Open No. 2010-268389.

As described above, the operation unit 46 is a portion that receives an input operation by the user, and includes a touch panel, operation buttons, and the like. Setting information such as the number of prints, whether or not double-sided printing is executed, the type of paper, and the like, which are input using the operation unit 46, and a print start instruction are input to the CPU 100.

In such an image forming apparatus 10, when the toner image (transferred image) on the paper is heat-fixed in the fixing unit 44, the heat causes the paper to shrink. For this reason, when performing double-sided printing, it is necessary to adjust the image size of the front and back sides to match, but as in the above-mentioned conventional technique, if the rotation speed of the polygon mirror is changed between the front side printing and the back side printing. , Printing productivity is reduced. Further, if the image data used for printing on the back surface side is reduced by image processing, there is a risk that image quality deterioration such as fine line omission may occur.

Therefore, in this first embodiment, when double-sided printing is performed, the rotation speed of the polygon mirror 82 is not changed between the front side printing and the back side printing, and the image data reduction process (compression process) is used. By forming the transferred image on the front side larger than the transferred image on the back side according to the shrinkage rate of the paper due to the heat fixing treatment, the image size of the front and back sides of the paper after the heat fixing treatment can be increased. I try to match it.

Hereinafter, the paper will be specifically described, but for the sake of simplicity, it is assumed that the paper shrinks by 3% in each of the main scanning direction and the sub scanning direction due to the heat fixing treatment. However, the shrinkage rate of this paper is an example, and in reality, it is preferable to perform the double-sided printing process described later according to the shrinkage rate of the paper to be printed. The shrinkage rate of the paper may be actually measured and set at the time of double-sided printing in the image forming apparatus 10, or may be stored in advance according to the type of paper and the like, and the type of paper by the user may be determined. It may be set according to the input information. It is also possible to set different values for the shrinkage ratio of the paper in the main scanning direction and the sub-scanning direction. Further, the image scaling operation described later is similarly executed for each color.

In this first embodiment, the size of the transferred image formed on the front surface side of the paper is equal to (100%) the size of the original image data in each of the main scanning direction and the sub scanning direction, and the size of the paper is increased. The double-sided printing process is performed so that the size of the transferred image formed on the back surface side is 97% of the original image data in each of the main scanning direction and the sub scanning direction.

Specifically, first, before printing on the front side, the rotation speed of the polygon mirror 82 is set to 103%, assuming that single-sided printing is 100%. As a result, the size of the electrostatic latent image (transferred image transferred to the paper) formed on the photoconductor drum 36 is 103% in the main scanning direction and 97% in the sub-scanning direction. It becomes the size.

Then, at the time of surface printing, the frequency (speed) of the printing clock is set to 103%. As a result, the size of the transferred image in the main scanning direction is reduced by 3%, so that the size is 100% when combined with the scaling factor (3% enlargement) by the polygon mirror 82. Further, the sub-scanning direction is enlarged by 3% by image processing in the image processing unit 112. As a result, the size of the transferred image in the sub-scanning direction becomes 100% when combined with the scaling factor (3% reduction) by the polygon mirror 82.

On the other hand, at the time of backside printing, the frequency of the printing clock is set to 106%. As a result, the size of the transferred image in the main scanning direction is reduced by 6%, so that the size is 97% when combined with the scaling factor (3% enlargement) by the polygon mirror 82. In addition, the sub-scanning direction is the same magnification processing without performing scaling by image processing. As a result, the size of the transferred image in the sub-scanning direction is changed by only the scaling factor (3% reduction) by the polygon mirror 82, so that the size is 97%.

By the double-sided printing process as described above, the image size on the front side can be reduced without changing the rotation speed of the polygon mirror 82 between the front side printing and the back side printing, and without using the image data reduction processing. The size can be 100%, and the image size on the back side can be 97%. As a result, when the paper shrinks by 3% in each of the main scanning direction and the sub scanning direction due to the heat fixing treatment on the front surface side, the image sizes of the front surface side and the back surface side are matched. Then, for example, in the case of paper that shrinks due to the heat of the heat fixing process and then returns to its original size by absorbing moisture again, it is the same as the original image data after the paper returns to its original size. Images of the size will be formed on the front and back.

In this first embodiment in which the size of the transferred image formed on the front surface side is the same size as the original data, the rotation speed of the polygon mirror 82 is set to 103% as described above when printing on the back surface. This is because the image processing in the scanning direction is the same size processing. That is, if the rotation speed of the polygon mirror 82 is set to less than 103%, it is necessary to reduce the image data in the sub-scanning direction at the time of backside printing, which causes fine line omission. On the other hand, when the rotation speed of the polygon mirror 82 is set to a size exceeding 103%, it is necessary to increase the enlargement ratio of the image data in the sub-scanning direction, especially at the time of surface printing. In the case of the enlargement processing, the effect on the image quality is small as compared with the reduction processing, but the image quality deteriorates as the enlargement ratio increases. Therefore, it is preferable to keep the enlargement ratio of the image by image processing as low as possible. Therefore, it is preferable that the rotation speed of the polygon mirror 82 is set so that the image processing in the sub-scanning direction becomes the same magnification processing at the time of backside printing. However, the rotation speed of the polygon mirror 82 can be set so that the image processing in the sub-scanning direction at the time of backside printing becomes the enlargement processing as long as it does not affect the image quality.

The operation in the image forming apparatus 10 as described above is realized by the CPU 100 of the control unit 28 executing a control program (double-sided printing program or the like) stored in the RAM 102. Hereinafter, the double-sided printing process in the image forming apparatus 10 will be described with reference to the flow chart shown in FIG.

When the user inputs an execution instruction for double-sided printing, the CPU 100 starts the double-sided printing process as shown in FIG. 5, and sets the rotation speed of the polygon mirror 82 in step S1. For example, the rotation speed of the polygon mirror 82 is set to be 103%. In the next step S3, the frequency of the printing clock for the surface is set in consideration of the rotation speed of the polygon mirror 82. For example, the frequency of the print clock is set to 103% by f-PLL. In the following step S5, the image data for the surface is enlarged in the sub-scanning direction by image processing in consideration of the rotation speed of the polygon mirror 82. For example, a control signal is output to the image processing unit 112 to enlarge the image data for the surface by 3%. Then, the CPU 100 outputs a control signal to the LD data generation unit 110 to generate a drive signal for the surface of the laser diode 72 according to the print clock and image data generated in steps S3 and S5.

In the following step S7, it is determined whether or not the rotation of the polygon mirror 82 is stable. That is, it is determined whether or not the polygon mirror 82 is stably rotating at the rotation speed set in step S1. If "NO" is set in step S7, that is, if the rotation of the polygon mirror 82 is not stable, the process waits until the rotation of the polygon mirror 82 becomes stable. When the polygon mirror 82 is already rotating at the rotation speed set in step S1, such as the second and subsequent sheets when performing double-sided printing on a plurality of sheets, this waiting time does not occur.

On the other hand, if "YES" in step S7, that is, if the rotation of the polygon mirror 82 is stable, the process proceeds to step S9 to execute the printing process on the front surface side. That is, the CPU 100 forms a two-dimensional electrostatic latent image on the surface of each photoconductor drum 36 that is rotationally driven by emitting a light beam from each laser diode 72 while rotating the polygon mirror 82. After that, a toner image is formed on each photoconductor drum 36, the toner image is transferred to the paper via the intermediate transfer belt 54, the secondary transfer roller 62, and the like, and the transferred image is transferred to the paper in the fixing unit 44. Heat-fixed.

When the printing process on the front surface side of step S9 is completed, in the following step S11, the rotation speed of the polygon mirror 82, the frequency of the printing clock for the surface, and the shrinkage rate (3%) of the paper due to the heat fixing process are taken into consideration. Set the frequency of the print clock for the back side. For example, the frequency of the print clock is set to 106% by f-PLL. In the following step S11, the enlargement magnification in the sub-scanning direction of the image data on the back surface side is set in consideration of the rotation speed of the polygon mirror 82 and the shrinkage rate of the paper due to the heat fixing process. For example, the magnification is set to 1x processing. Then, the CPU 100 outputs a control signal to the LD data generation unit 110 to generate a drive signal for the back surface of the laser diode 72 according to the print clock and image data generated in steps S11 and S13.

Then, in step S15, the printing process on the back side is executed in the same manner as in step S9. As a result, a transferred image is formed on the back side of the paper, which is reduced by 3% in each of the main scanning direction and the sub-scanning direction as compared with the front side, so that the shrinkage of the paper is absorbed and the paper is absorbed. The image sizes on the front and back are matched.

As described above, according to the first embodiment, the rotation speed of the polygon mirror 82 is not changed between the front side printing and the back side printing, that is, the mechanical operation is not changed. The time required for stabilization can be omitted. In addition, the image data reduction process is not used. Therefore, it is possible to prevent omission of fine lines and to appropriately match the image sizes of the front and back surfaces without lowering the printing productivity.

Further, according to the first embodiment, in the sub-scanning direction, the rotation speed of the polygon mirror is set so that the back side of the paper is processed at the same magnification, so that the enlargement ratio of the image by the image processing can be suppressed to a low level. .. Therefore, deterioration of image quality can be suppressed.

In particular, this first embodiment is suitable for forming an image on paper that returns to its original size by absorbing moisture again after shrinking due to the heat of the heat fixing treatment. Further, it is suitable when a problem occurs when the image on the front surface side is enlarged, for example, when the image on the front surface side is enlarged and the image is out of the effective print range.

[Second Example]
Next, the image forming apparatus 10 which is the second embodiment of the present invention will be described. In this second embodiment, the enlargement ratio of the image size on the front surface side is different from that in the first embodiment described above. Since the configurations of the other parts are the same, the same reference numbers will be described for the parts common to the first embodiment described above, and the duplicated description will be omitted or simplified.

In this second embodiment, the size of the transferred image formed on the front surface side of the paper is 103% of the original image data in each of the main scanning direction and the sub scanning direction, and is formed on the back surface side of the paper. The double-sided printing process is performed so that the size of the transferred image to be transferred is 100% of the original image data in each of the main scanning direction and the sub scanning direction.

Specifically, in the second embodiment, the rotation speed of the polygon mirror 82 is set to 100% even in the case of double-sided printing, as in the case of single-sided printing. That is, the polygon mirror 82 does not change the size of the electrostatic latent image formed on the photoconductor drum 36.

Then, at the time of surface printing, the frequency of the printing clock is set to 97%. As a result, the size of the transferred image in the main scanning direction is enlarged by 3% to a size of 103%. Further, the sub-scanning direction is enlarged by 3% by image processing by the image processing unit 112. As a result, the size of the transferred image in the sub-scanning direction becomes 103%.

On the other hand, at the time of backside printing, the frequency of the printing clock is set to 100%. As a result, the size of the transferred image in the main scanning direction becomes 100%. In addition, the sub-scanning direction is the same magnification processing without performing scaling by image processing. As a result, the size of the transferred image in the sub-scanning direction becomes 100%.

By the double-sided printing process as described above, the image size on the front side can be reduced without changing the rotation speed of the polygon mirror 82 between the front side printing and the back side printing, and without using the image data reduction processing. The size can be 103%, and the image size on the back side can be 100%. As a result, when the paper shrinks by 3% in each of the main scanning direction and the sub-scanning direction due to the heat fixing treatment on the front surface side, the image sizes of the front and back sides of the paper are matched. Then, for example, in the case of paper that returns to its original size after a predetermined time of heat shrinkage, when the paper returns to its original size, an image having a size of 103% of the original image data is formed on the front and back sides. Will be done. Further, for example, in the case of paper that does not return to its original size once it shrinks due to the heat of the heat fixing process, an image having the same size as the original image data is formed on the front and back sides.

Also in this second embodiment, as in the first embodiment, the rotation speed of the polygon mirror 82 is not changed between the front side printing and the back side printing, and the image data reduction process is not used. , The image size of the front side and the back side can be matched. Therefore, it is possible to prevent omission of fine lines and to appropriately match the image sizes of the front and back surfaces without lowering the printing productivity.

In particular, this second embodiment is suitable for paper that does not return to its original size when heat-shrinked, or has a small return rate. Further, it is suitable when there is no problem even if the image is enlarged and formed.

[Third Example]
Subsequently, the image forming apparatus 10 which is the third embodiment of the present invention will be described. In this third embodiment, the enlargement ratio of the image size on the front surface side is different from that of the first embodiment and the second embodiment described above. Since the configurations of the other parts are the same, the same reference numbers will be described for the parts common to the above-mentioned first embodiment and the second embodiment, and the duplicated description will be omitted or simplified.

In this third embodiment, the size of the transferred image formed on the front surface side of the paper is 106% of the original image data in each of the main scanning direction and the sub scanning direction, and is formed on the back surface side of the paper. The double-sided printing process is performed so that the size of the transferred image to be transferred is 103% of the original image data in each of the main scanning direction and the sub scanning direction.

Specifically, first, before printing on the front side, the rotation speed of the polygon mirror 82 is set to 97%, assuming that single-sided printing is 100%. As a result, the size of the electrostatic latent image formed on the photoconductor drum 36 becomes 97% in the main scanning direction and 103% in the sub-scanning direction.

Then, at the time of surface printing, the frequency of the printing clock is set to 91%. As a result, the size of the transferred image in the main scanning direction is enlarged by 9%, so that the size is 106% when combined with the scaling factor (3% reduction) by the polygon mirror 82. Further, the sub-scanning direction is enlarged by 3% by image processing in the image processing unit 112. As a result, the size of the transferred image in the sub-scanning direction becomes 106% when combined with the scaling factor (3% enlargement) by the polygon mirror 82.

On the other hand, at the time of backside printing, the frequency of the printing clock is set to 94%. As a result, the size of the transferred image in the main scanning direction is enlarged by 6%, so that the size is 103% when combined with the scaling factor (3% reduction) by the polygon mirror 82. In addition, the sub-scanning direction is the same magnification processing without performing scaling by image processing. As a result, the size of the transferred image in the sub-scanning direction is changed by only the scaling factor (3% enlargement) by the polygon mirror 82, so that the size is 103%.

By the double-sided printing process as described above, the image size on the front side can be reduced without changing the rotation speed of the polygon mirror 82 between the front side printing and the back side printing, and without using the image data reduction processing. The size can be 106%, and the image size on the back side can be 103%. As a result, when the paper shrinks by 3% in each of the main scanning direction and the sub scanning direction due to the heat fixing treatment on the front surface side, the image sizes of the front surface side and the back surface side are matched. Then, for example, in the case of paper that returns to its original size after a predetermined time of heat shrinkage, when the paper returns to its original size, an image having a size of 106% of the original image data is formed on the front and back sides. Will be done. Further, for example, in the case of paper that shrinks at the time of heat fixing on the back side as well as at the time of heat fixing on the front side and does not return to the original size when heat shrinks, an image having the same size as the original image data Will be formed on the front and back.

In the third embodiment as well, as in the first embodiment, the rotation speed of the polygon mirror 82 is not changed between the front side printing and the back side printing, and the image data reduction process is not used. , The image size of the front side and the back side can be matched. Therefore, it is possible to prevent omission of fine lines and to appropriately match the image sizes of the front and back surfaces without lowering the printing productivity.

Table 1 below summarizes the image scaling operations in the first to third embodiments as described above.

[Table 1]

[Fourth and Fifth Examples]
In each of the above-described embodiments, the enlargement ratio of the image size on the front surface side is set to a fixed value in advance. That is, in the first embodiment, the front surface side is 100% printed, in the second embodiment, the front surface side is 103% printed, and in the third embodiment, the front surface side is 106% printed, but the present invention is not limited to this.

As a fourth embodiment of the present invention, the user may arbitrarily select the enlargement ratio of the image size on the front surface side. For example, when a selection operation indicating the enlargement ratio of the image size on the front surface side is input by the user from the operation unit 46 or an external computer or the like, the CPU 100 of the control unit 28 determines the image size on the front surface side based on the selection operation. Set the magnification.

Further, as a fifth embodiment of the present invention, the enlargement ratio of the image size on the front surface side may be automatically changed based on the type of paper. In this case, the image forming apparatus 10 includes a paper discriminating unit (not shown) for discriminating the type of paper, and the CPU 100 of the control unit 28 is on the front side based on the type of paper discriminated by the paper discriminating unit. Set the enlargement ratio of the image size. For example, for paper that returns to its original size due to moisture absorption after heat shrinkage, set the image size so that the front side is 100% printed, and use paper that does not return to its original size after heat shrinkage of 3%. On the other hand, it is advisable to set the image size so that the front side is printed at 103%. Further, the paper discriminating unit discriminates the type of paper based on, for example, input information by the user.

In each of the above-described embodiments, in the main scanning direction, the transfer image on the front side is larger than the transfer image on the back side by changing the frequency of the print clock, but the transfer image on the front side is larger than the transfer image on the back side. Also, the image data may be enlarged so that the transferred image on the front surface side is larger than the transferred image on the back surface side.

In addition, the specific numerical values, materials, shapes, etc. mentioned above are merely examples, and can be appropriately changed as necessary, such as product specifications.

10 ... Image forming device 12 ... Device main body 14 ... Image reading device 28 ... Control unit 30 ... Image forming unit 32 ... Exposure device (exposure unit)
34 ... Developer 36 ... Photoreceptor drum (image carrier)
40 ... Charger 42 ... Transfer unit 44 ... Fixing unit 46 ... Operation unit 72 ... Laser diode (laser emission unit)
82 ... Polygon mirror 108 ... PM driver 110 ... LD data generation unit 112 ... Image processing unit

Claims (4)

An image forming apparatus having a double-sided printing function, in which a toner image formed on an image carrier is transferred to paper and then heat-fixed.
An exposed unit that has a laser emitting unit and a polygon mirror and forms an electrostatic latent image on the image carrier by scanning an optical beam based on image data.
An image processing unit capable of enlarging the image data at least in the sub-scanning direction, and a heat fixing process without changing the rotation speed of the polygon mirror between front side printing and back side printing when double-sided printing is performed. In the sub-scanning direction, the image data is enlarged by the image processing unit so that the transferred image on the front surface side becomes larger than the transferred image on the back surface side according to the shrinkage rate of the paper. Further, in the main scanning direction, a control unit for matching the image sizes of the front side and the back side of the paper by changing the frequency of the printing clock is provided.
The control unit enlarges the image size on the back side based on the set value of the enlargement ratio of the image size on the front side and the difference in the shrinkage ratio of the paper between the front side printing and the back side printing by the heat fixing process. Even when the enlargement ratio of the image size on the front side is arbitrarily set based on the ratio and the set value when the magnification of the image processing in the sub-scanning direction at the time of backside printing is set to 1x or enlargement. An image forming apparatus that sets the rotation speed of the polygon mirror to the same value during front side printing and backside printing. Wherein, based on a selection operation by the user, it sets the magnification of the image size of the surface side, an image forming apparatus according to claim 1. A paper discriminating unit for discriminating the type of paper
Wherein, based on the type of the paper is determined by the sheet discriminator, sets the magnification of the image size of the surface side, an image forming apparatus according to claim 1. It is provided with an exposed portion that forms an electrostatic latent image on the image carrier, and has a double-sided printing function that transfers a toner image formed on the image carrier based on the electrostatic latent image to paper and then heat-fixes the toner image. A control program executed by a computer of an image forming apparatus, wherein the computer is used.
An image processing unit capable of enlarging image data at least in the sub-scanning direction, and the paper by heat fixing processing without changing the rotation speed of the polygon mirror between front side printing and back side printing when double-sided printing is performed. In the sub-scanning direction, the image data is enlarged by the image processing unit so that the transferred image on the front surface side becomes larger than the transferred image on the back surface side according to the shrinkage rate of the image. In the main scanning direction, by changing the frequency of the print clock, it functions as a control unit that matches the image sizes of the front side and the back side of the paper.
The control unit enlarges the image size on the back side based on the set value of the enlargement ratio of the image size on the front side and the difference in the shrinkage ratio of the paper between the front side printing and the back side printing by the heat fixing process. Even when the enlargement ratio of the image size on the front side is arbitrarily set based on the ratio and the set value when the magnification of the image processing in the sub-scanning direction at the time of backside printing is set to 1x or enlargement. A control program that sets the rotation speed of the polygon mirror to the same value between front side printing and back side printing.

Images